The present invention relates to a bed for an electrical discharge machining apparatus, and particularly to a bed which results in improved machining accuracy.
FIG. 1 depicts a conventional electrical discharge machining apparatus in which a column 2 supporting a machining head 8 is fixed to a machine bed 1. A pair of saddle guide rails 3, provided on the bed 1, movably support a saddle 4 thereon. A table 6 is movably mounted on a pair of table guide rails 5 on the saddle 4. A workpiece 7 can be selectively fixed to the table 6. The workpiece 7 can thus be selectively moved in the X-Y plane, while the head 8 can be selectively moved in the Z direction.
The internal structure of the bed 1 is shown in FIG. 2. The bed 1 has a central reinforcing rib 11 arranged perpendicularly to the saddle guide rails 3, that is, in the Y direction.
To machine the workpiece 7, it is necessary to maintain the gap between the head 8 and the workpiece 7 constant while the workpiece is moved in the X-Y plane. During this movement in the X-Y plane, the centers of gravity of the workpiece 7, the saddle 4 and the table 6 shift relative to one another. The reinforcing rib 11 is provided to prevent the surface of the bed 1 from being deformed due to this shifting phenomenon, to thereby maintain the gap between the head 8 and the workpiece 7 constant. However, when the saddle 4 passes over the reinforcing rib 11, still some deformation, represented by .delta..sub.1 in FIG. 3, unavoidably occurs. This results in an inaccuracy in the gap between the head 8 and the workpiece 7, thereby limiting the accuracy of the machining operation.